Axial flow turbine meters are widely used for measuring flow of gas by which to determine gas usage and the like. To this end, an axial flow turbine meter includes a meter body having an inlet end connected to a gas supply line or pipe and an outlet end which may be connected to a gas delivery line or pipe. Inside the meter body at the inlet end is a cone or bullet-shaped inlet body, the exit end of which is placed adjacent to a turbine rotor assembly having at least first rotor which rotates in response to gas flowing thereover so as to provide a measure of the gas flow. A second rotor may be positioned downstream of the first rotor for more accurate or calibrating measurements. Blades on the rotor periphery cause the rotor to rotate. To focus the flowing gas on the blades, the inlet body conditions the flowing gas to change it from the space of the supply pipe to the annular path.
In order to provide reliable measurements, certain correction factors must be taken into account. In particular, the pressure and temperature of the gas passing through the meter are usually not equal to the standard pressure and temperature upon which gas suppliers base their pricing structure, and therefore the actual volume of gas indicated by the meter is corrected to standard conditions in order to determine the total price for the gas used.
It is desirable for a gas flow meter to measure flow volume with a high degree of accuracy, since flow volume errors of as little as a half a percent, occurring over a significant time period, can result in a substantial revenue error in the total gas usage. The gas supplier accordingly may significantly overcharge or undercharge the user. It will also be appreciated that a gas meter of a given design may be placed in a variety of types of installations whose usage rates may vary substantially from each other, and furthermore, there frequently is a significant variation in usage rates even for a particular installation as demand rises and falls in accordance with need. Thus, the goal for the designer of a gas flow meter is to design a meter which has a high degree of accuracy and repeatability over a wide range of flow rates.
In order to achieve accurate measurement of gas flow, it is standard practice to calibrate each meter by comparing the meter's indicated flow volume to a known flow volume and repeating this test over a range of flow rates so as to develop a "calibration curve" representing the meter's flow volume error in percent. One calibration method commonly used is to place the meter to be calibrated in series with a highly accurate flow measurement device such as a "flow prover" in a test line and to flow gas through the test line. At a given test point, the actual flow volumes of the test meter and the flow prover are acquired and each is corrected to standard conditions (e.g., 1 bar pressure and 15 degrees C.) based on measurements of the pressure and temperature of the gas entering the test meter and the pressure and temperature of the gas entering the flow prover. This test is repeated over a range of flow rates to arrive at the test meter's calibration curve. Using this curve, for any indicated flow rate, an accurate flow volume can be determined.
Thus, it will be appreciated that an accurate measurement of gas flow volume is highly dependent on accurate measurement of the pressure of the gas entering the meter, both at the time of meter calibration and at the time of usage. For instance, a pressure measurement error of one inch of water can translate into a 0.25 percent error in a meter's calibration curve.
During field use, one of the factors affecting the accuracy of a meter's calibration is variation in the configuration of the environment in which the meter is installed, from one installation to another. In one known turbine meter, the turbine rotor assembly and the inlet body form a complete or self-contained unit or module which is removable as a unit from the meter body for maintenance or calibration. The combined inlet body and rotor assembly are contained within a cylindrical sub-housing that is inserted axially into the meter body through the inlet end thereof. An annular pressure space is formed between the sub-housing and meter body radially outwardly of the inlet body. The pressure space is sealed from the rest of the meter so as to be confined over the inlet body and one or more apertures are formed in the sub-housing over the inlet body by which to permit pressure communication from the flow path over the inlet body to the pressure space. A sensor coupled to the pressure space may be used to obtain a readout of the pressure therein.
The plurality of apertures through the sub-housing require manufacturing time for machining, deburring, inspection, and testing. Moreover, in order to create the pressure space, the module must include the sub-housing. Thus, replacement of a bad rotor assembly results in disconnecting the gas line(s) from the meter body to remove the module and necessitates removal of the entire module, along with the inlet body, the latter being a component that is not prone to failure. The result is a less efficient and more costly system to build and maintain.
In other turbine gas meters, the inlet body is not part of the removable measuring module, but rather remains with the meter body, since it is a stationary part which is durable and seldom requires maintenance or replacement. For instance, in a prior meter design by the assignee of the present application, a measuring module including a separate turbine rotor assembly is removable from and insertable into an internal plenum within the body and adjacent to an exit end of the inlet body through a lateral opening in the body. The measuring module includes a main rotor which is carried within a generally cylindrical main rotor carrier and a sensing rotor which is carried within a generally cylindrical sensing rotor carrier, the two rotor carriers being connected to each other and to a top plate which covers the lateral opening in the body and supports a mechanical counter mechanism as well as connections for various sensors. To remove the module, the top plate is disconnected from the body, and the entire top plate and rotor assembly is removed, without having to disconnect the body from the gas supply line. The body includes an inlet body in the form of a nose cone with flow straightening vanes, but these remain with the body when the measuring module is removed.
With the stand-alone rotor assembly, calibration of the measuring module is performed independent of the inlet body and flow straightening vanes. Further, the measuring module can be removed from the body without disconnecting the body from the gas supply line. However, because the inlet body may have a substantial effect on the flow conditions, such as the static pressure, at the inlet to the measuring module, it is necessary to accurately measure and account for the static pressure during calibration and field use of the measuring module. To this end, pressure measured in the inlet flowpath above the inlet body is not a reliable or accurate source for calibration of the module since the module and inlet body are independent. Thus if a measuring module is calibrated in a test setup, the module may not be accurately calibrated for field use where the inlet body and other aspect of the meter body in the field may vary slightly from the test set-up unit. For example, the location of the sensor may vary from meter to meter or the size and spacing of the components may vary slightly from meter to meter. As a consequence, there is a marked risk of degraded repeatability and interchangeability of measuring modules from one meter body to another.
To overcome problems associated with measuring pressure over the inlet body, pressure measurements have been focused on the rotor turbine module assembly itself. To this end, pressure is measured at the inlet to the main rotor via a pressure tap machined through the main rotor carrier just upstream of the main rotor blades. The pressure tap is connected by flexible tubing to a fitting that extends through the top plate. A pressure sensor may be connected to the fitting. Thus, the pressure tap location is consistent from meter to meter and moves with the module. By measuring pressure close to the main rotor within the rotor assembly and making the pressure measurement system part of the removable measuring module, the same calibration curve can be used for the measuring module even when it is placed in different bodies.
This pressure measurement system has been found to be satisfactory up to gas flow velocities of about 110 feet per second. However, for reasons that are not known, it has been discovered that the accuracy of the pressure measured within the rotor assembly ahead of the main rotor begins to deteriorate at higher flow rates. For example, at 160 feet per second flow velocity, the meter calibration curve determined by using this pressure measurement is about 0.5 percent in error. Additionally, it has been found that pressure measurements taken over the inlet body exhibit inconsistencies with different measuring module/body combinations.
Accordingly, there has been a need for a pressure measurement system for a turbine flow meter module in which pressure can be accurately measured for a wide range of flow rates and which permits interchanging the meter in various meter bodies without having to recalibrate the meter each time it is placed in a new meter body.